A rechargeable battery, storage battery, secondary cell, or accumulator is a type of electrical battery which can be charged, discharged into a load, and recharged many times, while a non-rechargeable or primary battery is supplied fully charged, and discarded once discharged. Rechargeable batteries are composed of one or more electrochemical cells. The term “accumulator” is used as it accumulates and stores energy through a reversible electrochemical reaction. Rechargeable batteries are produced in many different shapes and sizes, ranging from button cells to megawatt systems connected to stabilize an electrical distribution network. Several different combinations of electrode materials and electrolytes are used, including lead-acid, nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer).
Rechargeable batteries are used for many applications including powering automobiles, portable consumer devices, light vehicles (such as motorized wheelchairs, golf carts, electric bicycles, and electric forklifts), tools, and uninterruptible power supplies. Emerging applications in hybrid internal combustion-battery and electric vehicles are driving the technology to reduce cost, weight, size, and increase lifetime. Grid energy storage applications use rechargeable batteries for load-leveling, storing electric energy at times of low demand for use during peak periods, and for renewable energy uses, such as storing power generated from photovoltaic arrays during the day to be used at night. Load-leveling reduces the maximum power which a plant must be able to generate, reducing capital cost and the need for peaking power plants.
Rechargeable batteries include a positive active material, a negative active material and in some cases an electrolyte. The positive active material and the negative active material are disposed in the electrolyte. During charging, the positive active material is oxidized, producing electrons, and the negative material is reduced, consuming electrons. These electrons constitute a current flow in a circuit external to the rechargeable battery. The electrolyte may serve as a buffer for internal ion flow between the electrodes, as in lithium-ion and nickel-cadmium cells, or the electrolyte may be an active participant in the electrochemical reaction, as in lead-acid cells.
The energy used to charge rechargeable batteries usually comes from a battery charger using AC mains electricity, or an alternator driven by a separate motive source such as an engine. Regardless of the source of energy, to store energy in a rechargeable battery, the rechargeable battery has to be connected to a DC voltage source. This is accomplished by connecting a negative terminal of the rechargeable battery to a negative terminal of a power source and a positive terminal of the power source to a positive terminal of the rechargeable battery. Further, a voltage output of the power source must be higher than that of the rechargeable battery, but not much higher: the greater the difference between the voltage of the power source and the battery's voltage capacity, the faster the charging process, but also the greater the risk of overcharging and damaging the rechargeable battery.
Battery charging and discharging rates are often discussed by referencing a “C” rate of current. The C rate is that which would theoretically fully charge or discharge the battery in one hour. For example, trickle charging might be performed at C/20 (or a “20 hour” rate), while typical charging and discharging may occur at C/2 (two hours for full capacity).
In some cases, rechargeable battery packs are formed of multiple electrochemical cells (hereinafter “cells”) that are connected together in a series or parallel configuration. The capacity within cells of the various rechargeable battery packs vary depending on the discharge rate. Some energy is lost in the internal resistance of cell components (plates, electrolyte, interconnections), and the rate of discharge is limited by the speed at which chemicals in the cell can move about. For lead-acid cells, the relationship between time and discharge rate is described by Peukert's law; a lead-acid cell that can no longer sustain a usable terminal voltage at a high current may still have usable capacity, if discharged at a much lower rate. Data sheets for rechargeable cells often list the discharge capacity on 8-hour or 20-hour or other stated time; cells for uninterruptible power supply systems may be rated at 15 minute discharge.
Battery manufacturers' technical notes often refer to voltage per cell (VPC) for the individual cells that make up the battery. For example, to charge a 12 V lead-acid battery (containing 6 cells of 2 V each) at 2.3 VPC requires a voltage of 13.8 V across the battery's terminals.
Cells can be placed in parallel with positive terminals of each cell electrically connected to positive terminals of other cells, and the negative terminals connected likewise. Cells can otherwise be placed in series where the negative terminal of one cell is connected to the positive terminal of another cell. Two cells in parallel are often denoted 2p, while two cells in series is denoted as 2s. This notation can then be chained left to right. For example, 2p2s are two cells in parallel in series with two other cells in parallel. Where 3p2s are 2 sets of 3 parallel cells placed in series for a total of six cells.
Two intermediate structures may arise when configuring cells into battery packs, a module and a section. A module is typically the smallest unit of the battery pack without breaking permanent mechanical systems. Battery sections are generally composed of a plurality of modules and may be structured for ease in disassembly and reconstituted through the use of removable hardware (e.g., threaded rods with removable nuts). These structures arise for two reasons. First is the requirement for mechanical compression which may be required for proper functioning. Second, intermediate electrical equipment, such as fuses and contactors, are positioned for safety and operation. For example, fuses are typically located mid-battery pack so that removal of the fuse reduces battery voltage by half.
Most battery chemistries may be capable of creating higher currents when warm. Additionally, providing a current may warm a battery. Higher temperatures of a battery pack may aid the battery packs in performance, however, such temperatures may also chemically degrade the battery pack overtime. The three main causes of reduction of capacity of an automotive battery are: heat, shelf life, and cycling. In passively cooled battery packs, heat may be a dominant reducer of battery life.
When batteries are positioned in a parallel configuration, one battery may be at a higher State-of-Charge (SOC) as compared to other batteries within the parallel configuration. The SOC is the percentage of the energy that the battery can accept or give relative to its capacity. The SOC is measured through the open circuit voltage (OCV) which is the voltage the battery would be at if the battery is not under current and at equilibrium. A battery at a higher SOC than the other batteries in parallel provides more current, which in turn warms the battery, providing even more current. This unstable condition is known as a thermal runaway. Proximity of adjacent battery cells allows for warmer battery cells to readily transmit heat via conduction to the adjacent battery cells. As the modules of the battery pack are in a series configuration, the module with the lowest capacity may set the over-all capacity since the string of modules in the series configuration will obtain the minimal, when discharging, and maximal, when charging, acceptable voltage first. In this way the weakest module sets the capacity for the entire pack. The potential lost capacity is the difference between the highest capacity cell and the lowest capacity cell. This loss of capacity within the pack creates capacity differences between packs, which in turn contributes to thermal runaway. A pack with a greater capacity is more likely to provide more current because its voltage stays higher as it discharges and lower as it charges than other packs on in parallel.
Two methods, both at great cost, try to remedy such issues. The first is an aggressive thermal management system typically liquid cooled. These systems may be effective at maintaining a uniform temperature throughout the battery pack, or between multiple liquid cooled packs. These systems, however, may have an increase in maintenance cost due to maintaining seals on a liquid cooling loop. Battery packs manufactured with liquid cooling may last longer, but are more expensive. Passively cooled (e.g., radiantly, air-cooled) battery packs may be less expensive to manufacture; however, these battery packs typically have a short design life. Passively cooled battery packs may be more available for use at a reduced cost; however, such systems may also need the use of a chiller unit consuming a significant amount of electricity and maintenance to monitor coolant levels.
The second approach for remedying lost capacity of a battery pack includes the use of DC/DC conversion electronics. In the prior art, however, a separate DC/DC converter is typically used for each battery pack, unlinking OCV, which is equivalent to the SOC, of the battery pack to the voltage of the DC bus. This allows each battery to produce a current independent of every other battery and to be controlled by separate ECUs. A battery pack having a higher OCV may generate more current. As such, if multiple battery packs are being used, the imbalance between voltage and current across the battery packs may cause thermal runaway problems.
Thus, in using multiple battery packs, it would be advantageous to be able to allow for manipulation of one or more battery packs or portions of a battery pack to reduce lost capacity. Manipulating a whole pack by removing it from the DC bus through switches, however, creates a situation where the OCV no longer matches the voltages of the other packs in parallel. When placing battery packs back on the DC bus, extra care may be needed such that the voltages are similar enough to prevent a reverse current through one of the packs, or a strong enough recirculating current isn't generated to melt the DC bus. Manipulating the whole pack while the pack is still attached to the DC bus also has the problem that it may manipulate every pack on the bus as manipulation may be only at the pack in interest in proportion to the DC resistance on the bus line. Since this is relatively low, all of the packs close to the pack being manipulated may also be manipulated creating a highly inefficient method.